Polarizing beam splitters providing high resolution images and systems utilizing such beam splitters

ABSTRACT

Polarizing beam splitters and systems incorporating such beam splitters are described. More specifically, polarizing beam splitters and systems with such beam splitters that incorporate multilayer optical films and reflect imaged light towards a viewer or viewing screen with high effective resolution are described.

FIELD

The present description relates to polarizing beam splitters and systemsincorporating such beam splitters. More specifically, the presentdescription relates to polarizing beam splitters and systems with suchbeam splitters that incorporate multilayer optical films and reflectimaged light towards a viewer or viewing screen with high effectiveresolution.

BACKGROUND

Illumination systems incorporating polarizing beam splitters (PBSs) areused to form images on viewing screens, such as projection displays. Atypical display image incorporates an illumination source that isarranged so that light rays from the illumination source reflect off ofan image-forming device (i.e., an imager) that contains the desiredimage to be projected. The system folds the light rays such that thelight rays from the illumination source and the light rays of theprojected image share the same physical space between a PBS and theimager. The PBS separates the incoming illumination light from thepolarization-rotated light from the imager. Due to new demands on PBSs,in part due to their new uses in applications such as, e.g.,three-dimensional projection and imaging, a number of new issues havearisen. The present application provides articles that address suchissues.

SUMMARY

In one aspect, the present description relates to a polarizationsubsystem. The polarization subsystem includes a first imager and apolarizing beam splitter. In some embodiments, the imager may be an LCOSimager. The polarizing beam splitter is made up in part of a reflectivepolarizer and receives imaged light from the imager. The reflectivepolarizer may be a multilayer optical film. In some embodiments, thereflective polarizer will have a surface roughness

Ra of less than 45 nm or a surface roughness Rq of less than 80 nm. Thepolarizing beam splitter reflects imaged light towards a viewer orscreen with an effective pixel resolution of less than 12 microns. Insome embodiments, the polarizing beam splitter may reflect imaged lighttowards a viewer or screen with an effective pixel resolution of lessthan 9 microns, or less than 6 microns. The polarization subsystem mayinclude a second imager, where the polarizing beam splitter receivesimaged light from the second imager at a different face from that whereit receives light from the first imager. The polarization subsystem mayalso include a projection lens that projects light from the polarizingbeam splitter towards a viewer or screen. In some cases, thepolarization subsystem may be part of a three-dimensional imageprojector.

In another aspect, the present description relates to a polarizing beamsplitter. The polarizing beam splitter includes a reflective polarizerthat is positioned between a first cover and a second cover. Thereflective polarizer may be a multilayer optical film. The polarizingbeam splitter is capable of reflecting imaged light towards a viewer orscreen with an effective pixel resolution of less than 12 microns, andpotentially less than 9 microns or less than 6 microns. The first and/orsecond covers of the polarizing beam splitter may be made, at least inpart, of glass or suitable optical plastic. The first and/or secondcovers may be attached to the reflective polarizer by a suitable opticaladhesive with additional processing, such as exposure to vacuum, toachieve the desired flatness of the multilayer optical film. Thereflective polarizer may have a surface roughness Ra of less than 45 nmor a surface roughness Rq of less than 80 nm.

In yet another aspect, the present description relates to a projectionsubsystem. The projection subsystem includes a light source, apolarizing beam splitter, at least a first imager, and potentially asecond imager. The polarizing beam splitter receives light from thelight source and includes a reflective polarizer made up of a multilayeroptical film. The first imager is positioned adjacent to the polarizingbeam splitter. The second imager is positioned adjacent to thepolarizing beam splitter on a different side of the polarizing beamsplitter than the first imager. Light from the light source is incidentupon the polarizing beam splitter and a first polarization of incidentlight is transmitted through the reflective polarizer while a secondpolarization of incident light orthogonal to the first polarizationstate is reflected by the reflective polarizer. Light of the secondpolarization travels from the polarizing beam splitter to the secondimager and is imaged and reflected back towards the polarizing beamsplitter. Light reflected from the second imager is transmitted throughthe polarizing beam splitter to an image plane. Light of the firstpolarization is transmitted through the polarizing beam splitter to thefirst imager and is imaged and reflected back towards the polarizingbeam splitter. Light reflected from the first imager is reflected at thepolarizing beam splitter towards an image plane with an effective pixelresolution of less than 12 microns. In at least some embodiments, lightreflected from the first imager is reflected at the polarizing beamsplitter towards an image plane with an effective resolution of lessthan 9 microns or less than 6 microns. The reflective polarizer may havea surface roughness Ra of less than 45 nm or a surface roughness Rq ofless than 80 nm. The light source of the projection subsystem may be anysuitable light source such as an arc lamp or an LED or LEDs.

In another aspect, the present description relates to a polarizationsubsystem. The polarization subsystem includes a first imager and apolarizing beam splitter. The polarizing beam splitter is made up inpart of a reflective polarizer and receives imaged light from theimager. The reflective polarizer may be a multilayer optical film. Thepolarizing beam splitter reflects imaged light towards a viewer orscreen. In some embodiments, the reflective polarizer has a surfaceroughness Ra of less than 45 nm or a surface roughness Rq of less than80 nm. In some embodiments, the reflective polarizer has a surfaceroughness Ra of less than 40 nm or a surface roughness Rq of less than70 nm. In some embodiments, the reflective polarizer has a surfaceroughness Ra of less than 35 nm or a surface roughness Rq of less than55 nm

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a polarization conversion system according to the presentdescription.

FIG. 2 is a polarizing beam splitter according to the presentdescription.

FIG. 3 is a projection subsystem according to the present description.

FIG. 4 is a flowchart illustrating a method of making a flat multilayeroptical film for use in a PBS.

FIG. 5 illustrates a method for creating a polarizing beam splitterusing a multilayer optical film.

DETAILED DESCRIPTION

A high performance PBS is essential for creating a viable optical enginefor a projector that uses Liquid Crystal on Silicon (LCOS) imagers. Inaddition, a PBS may be required even for nominally unpolarized imagerssuch as DLP imagers when such imagers are required to handle polarizedlight. Typically, a PBS will transmit nominally p-polarized light andreflect nominally s-polarized light. A number of different types of PBSshave been used, including MacNeille type PBSs and wire grid polarizers.However, PBSs based on multilayer optical film have proven to be one ofthe most effective polarizing beam splitters for issues associated withlight handling in projection systems, including the ability toeffectively polarize over a range of wavelengths and angles of incidenceand with high efficiencies both in reflection and transmission. Suchmultilayer optical films are made by 3M Company, as described in U.S.Pat. No. 5,882,774 to Jonza et al., and U.S. Pat. No. 6,609,795 to Weberet al.

With the advent of a number of new imaging and projection applications,including, e.g., three-dimensional projection and imaging, newchallenges have arisen. Specifically, in at least some three-dimensionalimaging applications, it may be required that a PBS provide imaged lightthat has a high effective resolution (as defined below) not only whentransmitted through a reflective polarizing film, but also whenreflected by a reflective polarizing film. Unfortunately, polarizersbased on multilayer optical film, despite their other major advantages,may be difficult to formulate with the requisite flatness to reflectimaged light at high resolution. Rather, where such multilayer filmreflective polarizers are used to reflect imaged light, the reflectedimage may be distorted. However, the concerns of effectively polarizinga wide array of angles of incident light and wavelengths of incidentlight must still be addressed. It would therefore be highly desirable toprovide a polarizing beam splitter that has the benefits of a PBS thatcontains multilayer optical film, while also achieving heightenedeffective resolution for imaged light reflected off of the PBS towards aviewer or screen. The present description provides such a solution.

FIG. 1 provides an illustration of one polarization subsystem accordingto the present description. Polarization subsystem includes a firstimager 102. In a number of embodiments, such as that illustrated in FIG.1, the imager will be an appropriate reflective imager. Often, imagersused in projection systems are typically polarization-rotating,image-forming devices, such as liquid crystal display imagers, whichoperate by rotating the polarization of the light to produce an imagecorresponding to digital video signals. Such imagers, when used inprojection systems, typically rely on polarizers to separate light intoa pair of orthogonal polarization states (e.g., s-polarization andp-polarization). Two common imagers that may be used in the embodimentshown in FIG. 1 include a liquid crystal on silicon (LCOS) imager, ordigital light processing (DLP) imager. Those skilled in the art willrecognize that the DLP system will require some modification to theillumination geometry as well as an external means of rotating thepolarization (such as a retarder plate) in order to make use of the PBSconfiguration shown in FIG. 1. The polarization subsystem also includesa polarizing beam splitter (PBS) 104. Light 112 from a light source 110travels towards PBS 104. Within PBS 104 is a reflective polarizer 106.The reflective polarizer may be a multilayer optical film such as thoseavailable from 3M Company (St. Paul, Minn.) and described in, e.g., U.S.Pat. No. 5,882,774 to Jonza et al., and U.S. Pat. No. 6,609,795 to Weberet al., each of which is hereby incorporated by reference in itsentirety. When light 112 is incident upon film 106, one orthogonalpolarization state of the incident light, such as the p-polarized state,will be transmitted through the film and exit the PBS as light 120 thatis then incident on imager 102. The orthogonal polarization state of theincident light (in this case, s-polarized light), will be reflected byreflective polarizer 106 as a separate beam 118 in a differentdirection, here at right angles to beam 120.

Unimaged light of a given polarization state 120 is incident upon imager102. The light is then imaged and reflected back towards PBS 104 andincorporated reflective polarizer 106. Where the imager 102 is an LCOSimager, and for those pixels in an “on” state, light 114 is alsoconverted to an orthogonal polarization state. In this case, thep-polarized incident light, not yet imaged, is reflected as imaged lightof s-polarization. When the s-polarized light is incident upon thepolarizing beam splitter 104, and particularly multilayer optical filmreflective polarizer 106, the light is reflected as s-polarized beam 116towards a viewer or viewing screen 130.

In a number of embodiments of the prior art, the imager may bepositioned, e.g., in the direction towards which beam 118 travels. Insuch an embodiment imaged light would be transmitted through thepolarizing beam splitter 104 rather than reflected in polarizing beamsplitter 104. Transmitting imaged light through the polarizing beamsplitter allows for less distortion of the image, and thus, highereffective resolution. However, as will be further explained, it may bedesirable in a number of embodiments to include an imager 102 aspositioned in FIG. 1. This may, for example, allow for overlappingimages of different polarizations. Despite the many benefits ofmultilayer optical film as a reflective polarizer, it has conventionallybeen difficult to achieve high effective resolution for imaged lightreflected off such films.

The Effective Resolution of the image or light produced by elements is auseful quantitative measurement because it helps predict what size pixelcan be reliably resolved. Most current imagers (LCOS and DLP) have apixel size range from about 12.5 μm down to around 5 μm. So in order tobe useful in a reflective imaging situation, the reflector must be ableto resolve down to at least about 12.5 μm, and ideally better. Thereforethe Effective Resolution of a PBS must be no more than about 12.5 μm,and preferably lower. This would be considered a high effectiveresolution.

Using techniques described in the specification, one may in fact providea multilayer optical film for use in a PBS 104 that can reflect imagedlight at very high resolution. In fact, looking to FIG. 1, imaged light116 may be reflected from the polarizing beam splitter 104 towards aviewer or viewing screen 130 with an effective pixel resolution of lessthan 12 microns. In fact, in some embodiments, the imaged light 116 maybe reflected from the polarizing beam splitter 104 towards a viewer orviewing screen 130 with an effective pixel resolution of less than 11microns, less than 10 microns, less than 9 microns, less than 8 microns,less than 7 microns, or potentially even less than 6 microns.

As discussed, in at least some embodiments, the polarization subsystem100 may include a second imager 108. Second imager 108 may generally beof the same type of imager as first imager 106, e.g., LCOS or DLP. Lightof one polarization state, such as s-polarized light, may be reflectedfrom PBS 104, and specifically from reflective polarizer 106 of the PBStowards the second imager. It may then be imaged and reflected backtowards PBS 104. Again, as with the first imager 104, light reflectedoff of second imager 108 is polarization converted, such that wheres-polarized unimaged light 118 is incident upon imager 108, p-polarizedimaged light 122 is redirected from the imager 108 back towards PBS 104.Whereas light 114 reflected from imager 102 is of a first polarizationstate (e.g., s-pol) and therefore reflects off of PBS 104 towards vieweror viewing screen 130, light reflected off of imager 108 (e.g. light122) is of a second polarization (e.g., p-pol.) and therefore istransmitted through PBS 104 towards viewer or viewing screen 130. As canbe seen from FIG. 1, the two imagers are located at different sides ofthe PBS 104, such that the PBS receives imaged light 114 from firstimager 102 at a first face 126 and receives imaged light 122 from thesecond imager 108 at a second face 124 different from the first face.

Once imaged light 116 and potentially light 122 exits PBS 104 it isdirected towards a viewer or viewing screen 130. In order to best directlight to the viewer and properly scale the image, light may be passedthrough a projection lens 128 or some sort of projection lens system.While only illustrated with a single element projection lens 128,polarization conversion system 100 may include additional imaging opticsas needed. For example, the projection lens 128 may in fact be aplurality of lenses, such as lens group 250 of commonly owned andassigned U.S. Pat. No. 7,901,083. Note that in the case that optionalimager 108 is not used, the input light 112 may be pre-polarized to havethe same polarization state as light beam 120. This can be accomplishedfor example, by the use of a polarization converting system (PCS), theaddition or a reflective or absorptive linear polarizer or other suchdevice for enhancing the polarization purity of the input light stream112. Such a technique may improve the overall efficiency of the system.

PBS 104 may include other elements besides reflective polarizer 106. Forexample, FIG. 1 illustrates a PBS 104 that also includes a first cover132 and a second cover 134. Reflective polarizer 106 is positionedbetween first cover 132 and second cover 134, such that it is bothprotected and properly positioned by the covers. The first cover 132 andsecond cover 134 may be made of any appropriate material known in theart, such as glass, plastic or potentially other appropriate materials.It should be understood that additional materials and constructions maybe applied to, e.g. the faces of the PBS or adjacent to andsubstantially coextensive with the reflective polarizer. Such othermaterials or constructions may include additional polarizers, dichroicfilters/reflectors, retarder plates, anti-reflection coatings, lensesmolded and/or bonded to the surface of the covers and the like.

Projection or polarization subsystems that emit light from differentimagers, wherein the imaged light is of different polarizations may beespecially useful as part of a three-dimensional image projector asdescribed for example in U.S. Pat. No. 7,690,796 (Bin et al.). Thedistinct advantage of using a PBS based two imager system is that notime sequencing or polarization sequencing is required. This means thatboth imagers are operating at all times, effectively doubling the lightoutput of the projector. As discussed, it is highly important that thereflective polarizer 106 be flat, such that the imaged light 116reflected off of the polarizer is not distorted and has high effectiveresolution. Flatness can be quantified by the standard roughnessparameters Ra (the average of the absolute value of the verticaldeviation of the surface from the mean), Rq (the root mean squaredaverage of the vertical deviation of the surface from the mean), and Rz(the average distance between the highest peak and lowest valley in eachsampling length). Specifically, the reflective polarizer preferably hasa surface roughness Ra of less than 45 nm or a surface roughness Rq ofless than 80 nm, and more preferably has a surface roughness Ra of lessthan 40 nm or a surface roughness Rq of less than 70 nm, and even morepreferably has a surface roughness Ra of less than 35 nm or a surfaceroughness Rq of less than 55 nm. One exemplary method of measuring thesurface roughness or flatness of the film is provided in the Examplessection below.

In another aspect, the present description relates to a polarizing beamsplitter. One such polarizing beam splitter 200 is illustrated in FIG.2. Polarizing beam splitter 200 includes a reflective polarizer 206 thatis positioned between a first cover 232 and a second cover 234. As withreflective polarizer 106 of FIG. 1, the reflective polarizer 206 of FIG.2 is a multilayer optical film such as those described above. Thepolarizing beam splitter 200 is capable of reflecting imaged light 216towards a viewer or surface 230. The effective pixel resolution of theimaged light 216 that is directed towards the viewer or surface is lessthan 12 microns, and possibly less than 11 microns, less than 10microns, less than 9 microns, less than 8 microns, less than 7 microns,or potentially even less than 6 microns.

As with the covers of FIG. 1, first cover 232 and second cover 234 ofPBS 200 may be made of any number of appropriate materials used in thefield, such as glass or optical plastics, among others. In addition, thefirst cover 232, and second cover 234 may each be attached to reflectivepolarizer 206 by a number of different means. For instance, in oneembodiment, the first cover 232 may be attached to the reflectivepolarizer 206 using a pressure sensitive adhesive layer 240. A suitablepressure sensitive adhesive is 3M™ Optically Clear Adhesive 8141(available from 3M Company, St. Paul, Minn.). Similarly, the secondcover 234 may be attached to the reflective polarizer using a pressuresensitive adhesive layer 242. In other embodiments, the first and secondcover may be attached to reflective polarizer 206 using differentadhesive types for layer 240 and 242. For example, layers 240 and 242may be made up of a curable optical adhesive. Suitable optical adhesivesmay include optical adhesives from Norland Products Inc. (Cranbury,N.J.), such as NOA73, NOA75, NOA76 or NOA78, the optical adhesivesdescribed in commonly owned and assigned U.S. Patent Publication No.2006/0221447 (to DiZio et al.) and commonly owned and assigned U.S.Patent Publication No. 2008/0079903 (to DiZio et al.), each of which ishereby incorporated by reference. UV curable adhesives may also be used.It should be understood that additional materials and constructions maybe applied to, e.g. the faces of the PBS or adjacent to andsubstantially coextensive with the reflective polarizer. Such othermaterials or constructions may include additional polarizers, dichroicfilters/reflectors, retarder plates, anti-reflection coatings, and thelike. As with the PBS described in FIG. 1, the reflective polarizer 206of FIG. 2 must be very flat to most effectively reflect imaged light 216without distorting it. The reflective polarizer may have a surfaceroughness Ra of less than 45 nm or a surface roughness Rq of less than80 nm. With typical application procedures of pressure sensitiveadhesives such as described in U.S. Pat. No. 7,234,816 B2 (Bruzzone etal.) the required surface flatness of the reflective polarizer is notachieved. It has been discovered that certain types of postprocessing,allow the required surface flatness to be achieved.

In yet another aspect, the present description relates to a projectionsubsystem. One such projection subsystem is illustrated in FIG. 3.Projection subsystem 300 includes a light source 310. Light source 310may be any number of appropriate light sources commonly used inprojection systems. For example, the light source 310 may be asolid-state emitter such as a laser or light emitting diode (LED)emitting light of a specific color such as red, green, or blue light.The light source 310 may also include a phosphor or other lightconverting material that absorbs light from the emissive source andre-emits light at other (generally longer) wavelengths. Suitablephosphors include well known inorganic phosphors such as Ce-doped YAG,strontium thiogallate, and doped silicate and SiAlON-type materials.Other light converting materials include III-V and II-VI semiconductors,quantum dots, and organic fluorescent dyes. Alternatively, the lightsource may be made up of a plurality of light sources, such as a red, agreen and a blue LED, where such LEDs may be activated together orsequentially. Light source 310 may also be a laser light source, orpotentially a traditional UHP lamp. It is to be understood thatancillary components such as color wheels, dichroic filters orreflectors and the like may additionally comprise light source 310.

The projection subsystem 300 further includes a polarizing beam splitter304. Polarizing beam splitter 304 is positioned such that it receiveslight 312 from the light source. This incident light 312 may generallybe made up in part of two orthogonal polarization states, e.g., parts-polarized light, and part p-polarized light. Within the polarizingbeam splitter is a reflective polarizer 306, again in this case amultilayer optical film such as those described with respect toreflective polarizer 106. Light 312 is incident upon reflectivepolarizer 306 and light of one first polarization, e.g., p-polarizedlight is transmitted through as light 320 while light of a secondorthogonal polarization, e.g. s-polarized light, is reflected as light318.

Light of the first polarization 320 that is transmitted through thereflective polarizer 306 travels towards a first imager 302 that ispositioned adjacent to the PBS 304. Light is imaged and reflected at thefirst imager 302 back towards PBS 304 with the polarization of the lightconverted. The converted imaged light 314 is then reflected at the PBS304 as light 316 towards an image plane 350. The light 316 is reflectedoff of the reflective polarizer 306 of the PBS and reaches image plane350 with an effective resolution of less than 12 microns, and possiblyless than 11 microns, less than 10 microns, less than 9 microns, lessthan 8 microns, less than 7 microns, or potentially even less than 6microns. The reflective polarizer 306 typically has a surface roughnessRa of less than 45 nm or a surface roughness Rq of less than 80 nm.

Light of the second polarization (e.g. s-polarized) light that isreflected initially by the reflective polarizer of PBS 304 travels aslight 318 towards a second imager 308. Second imager 308 is alsopositioned adjacent the PBS 304, as with first imager 302, but secondimager is positioned on a different side of the PBS. The incident light318 is imaged and reflected back towards PBS 304. Upon reflection fromthe imager, the polarization of this light is rotated as well by 90degrees (e.g. from s-polarized light to p-polarized light). The imagedlight 322 is transmitted through the PBS 304 to the image plane 350. Thefirst imager 302 and second imager 308 may be any appropriate type ofreflective imager, such as those described above with respect toelements 102 and 108 of FIG. 1.

As discussed, in order to achieve high effective resolution for imagedlight reflected off of the PBS herein, the reflective polarizer of thePBS must be exceptionally optically flat. The present description nowprovides methods of producing an optically flat reflective polarizerthat is a multilayer optical film and/or methods of producing anoptically flat polarizing beam splitter.

One such method is illustrated in the flowchart of FIG. 4. The methodbegins with providing a multilayer optical film 410, and providing aflat substrate 420. The multilayer optical film 410 may be similar tothe multilayer optical films described with respect to the articlesabove. The flat substrate may be any number of appropriate materials,such as acrylic, glass or other appropriate plastics. Most importantly,the substrate 420 must possess at least the same degree of opticalflatness as is required in the polarizing beam splitter and must allow awetting solution to spread over its surface. Therefore, other plastics,inorganic glasses, ceramics, semiconductors, metals or polymers may beappropriate materials. Additionally it is useful for the substrate to beslightly flexible.

In the next step, the surface 425 of the flat substrate is releasablyattached to a first surface of the multilayer optical film. In at leastone embodiment, in order to create a releasable attachment, either thesurface 425 of the flat substrate, or a first surface of the multilayeroptical film, or both is wetted with a wetting agent, resulting in athin layer of solution 430. A suitable wetting agent should have asurface energy that is sufficiently low that it will wet out thesubstrate or the film and a vapor pressure that is sufficiently highthat it can evaporate at room temperature. In some embodiments,isopropyl alcohol is used as the wetting agent. In at least someembodiments the wetting agent will be an aqueous solution that containsat least a small amount of surfactant (e.g. less than 1% by volume). Thesurfactant may be common commercially available industrial wettingagents, or even household materials such as dishwashing detergent. Otherembodiments may be aqueous mixtures of compounds that leave no residueupon evaporation such as ammonia, vinegar, or alcohol. The wetting agentmay be applied by a number of appropriate methods including spraying,e.g., from a spray bottle. In the next step, the multilayer optical filmis applied to the surface of the substrate 425 such that the solution430 is sandwiched between the film and substrate. Typically the wettingagent is applied to the contacting surface of the multilayer opticalfilm also. A pressure applying instrument 435, such as a squeegee isthen drawn across the top of multilayer optical film 410 closelyflattening optical film 410 to the surface 425 of substrate 420, andleaving only a thin, fairly uniform layer of solution 430 separating thetwo. In at least some embodiments, a protective layer may first beapplied to the multilayer optical film on the side opposite the surface440 that is applied to the substrate 420. At this point, theconstruction is left to allow the solution 430 to evaporate. Thesqueegeeing process pushes residual water past the edges of themultilayer optical film such that only a small amount remains. Next, themultilayer optical film, flat substrate, and wetting agent are allowedto dry. With time, all of the volatile components of the wettingsolution evaporate either through layers 410 or 420 or by wicking alongthe space between layers 410 and 420 to the edges of layer 410 whereevaporation can occur. As this process occurs, the multilayer opticalfilm 410 is drawn closer and closer to substrate 420 until layer 410closely conforms to the surface 425. The result is shown in the nextstep of FIG. 4, as the drying closely draws the film 410 to substrate420 and effectively flattens the bottom surface 440 of the multilayeroptical film. Once this flatness has been achieved, the multilayeroptical film 410 remains stably flat but releasably attached to thesubstrate. At this point a permanent substrate may be adhered to theexposed surface of the film 410.

FIG. 5 illustrates further steps that may be taken in providing a finalconstruction of a polarizing beam splitter. For example, an adhesive 550may be applied on the flattened surface 450 of film 410. The adhesivemay be any appropriate adhesive that does not adversely affect theoptical or mechanical performance of the PBS. In some embodiments, theadhesive may be a curable optical adhesive, such as NOA73, NOA75, NOA76or NOA78 from Norland Products Inc. (Cranbury, N.J.). In otherembodiments, optical epoxies may be used. In some embodiments, theadhesive may be a pressure sensitive adhesive. Next, one may provide apermanent second substrate. In one embodiment, the permanent secondsubstrate may be a prism. As shown in FIG. 5, Prism 560 is appliedagainst the adhesive 550 and the construction is cured if appropriate.The film 410 may now be removed from the substrate 420. In at least oneembodiment, the film 410 is peeled away from substrate 420, typically byflexing substrate 420 slightly to allow the film 410 to release fromsubstrate 420. For cured adhesives such as UV adhesives or epoxies thenewly exposed bottom surface of the film 440 retains the flatness of thesubstrate 420. For pressure sensitive adhesives, the bottom surface ofthe film 440 may retain the flatness of the substrate 420 or may requireadditional processing to maintain the flatness. Once the flat filmsurface 440 has been achieved a second layer of adhesive 570 may beapplied to the bottom surface of the film 440 and a second prism orother permanent substrate 580 may be applied to the adhesive. Again theconstruction may be cured as needed, resulting in a complete polarizingbeam splitter.

Another method of making an optically flat polarizing beam splitterincludes the use, specifically, of pressure sensitive adhesives. Withappropriate techniques, the multilayer optical film may be made toconform closely to the flat surface of the prism. The following stepsmay be included. First, a multilayer optical film is provided. Themultilayer optical film will act as a reflective polarizer. This may besimilar to reflective polarizer optical film 410 of FIG. 5 with theexception that the surface 440 may not be substantially flattenedalready through the steps shown in FIG. 4. A layer of pressure sensitiveadhesive (here corresponding to adhesive layer 550) may be applied tothe first surface 440 of the multilayer optical film. Next a prism 560may be applied against the pressure sensitive adhesive layer adhesivelayer on the side opposite the multilayer optical film 410. The methodmay also include applying a second layer of adhesive (e.g. layer 570) ona second surface 575 of the film opposite first surface 440. A secondprism 580 may then be applied to the opposite side of layer 570 fromfilm 410. The present method provides an improvement over this methodthat further enhances the flatness of the reflective polarizer/prisminterface, such that imaged reflection off of the PBS has enhancedresolution. After pressure sensitive adhesive 550 is applied between theprism 560 and multilayer optical film 410, the construction is subjectedto vacuum. This may occur, for example, by placing the construction in avacuum chamber equipped with a conventional vacuum pump. The vacuumchamber may be lowered to a given pressure, and the sample may be heldat that pressure for a given amount of time, e.g., 5-20 minutes. Whenair is re-introduced to the vacuum chamber, the air pressure pushes theprism 560 and multilayer optical film 410 together. Where a secondadhesive layer and second prism are also applied, the subjection tovacuum in the chamber may optionally be repeated for the secondinterface (e.g. at layer 570). Applying vacuum to a prism/MOF assemblyresults in a PBS that provides heightened effective resolution whenimaged light is reflected off of the PBS. In place of or in conjunctionwith the vacuum treatment, a thermal/pressure treatment may also beused. It may be advantageous to conduct the processing more than onetime.

EXAMPLES

The following list of materials and their source is referred tothroughout the Examples. If not otherwise specified, materials areavailable from Aldrich Chemical (Milwaukee, Wis.). Multilayer OpticalFilms (MOFs) were generally prepared according to methods described in,for example, U.S. Pat. No. 6,179,948 (Merrill et al); U.S. Pat. No.6,827,886 (Neavin et al); 2006/0084780 (Hebrink et al); 2006/0226561(Merrill et al.); and 2007/0047080 (Stover et al.).

Roughness Measurement Method

Prisms were placed on modeling clay and leveled using a plunger leveler.Topographic maps were measured with a Wyko® 9800 optical interferometer(available from Veeco Metrology, Inc., Tucson, Ariz.), with a 10×objective and 0.5× field lens and with the following settings: VSIdetection; 4 mm×4 mm scan area stitched using 6 rows and 5 columns ofindividual maps, 2196×2196 pixels with a sampling of 1.82 μm; tilt andsphere correction used; 30-60 microns back scan length with 60-100forward scan length; with the modulation detection threshold 2%.Autoscan detection was enabled at 95% with 10 μm post scan length (thisshort post scan length avoided subsurface reflections in the datacollection).

A 4 mm×4 mm area in the central region of the hypotenuse-face of eachprism was measured. Specifically, the topography of each region wasmeasured, plotted, and the roughness parameters Ra, Rq and Rz werecalculated. One measurement area was obtained per prism. Three prismsamples were measured in each case and the mean and standard deviationof the roughness parameters were determined.

Example 1: Wet Application Method

A reflective polarizing multilayer optical film (MOF) was releasablydisposed onto an optically flat substrate in the following manner. Firsta wetting solution comprising approximately 0.5% mild dishwashingdetergent in water was placed into a spray bottle. A sheet ofapproximately 6 mm high-gloss acrylic was obtained and the protectivelayer removed from one side in a clean hood. The exposed acrylic surfacewas sprayed with the wetting solution so that the entire surface waswet. Separately a piece of MOF was obtained and one of its skin layerswas removed in a clean hood. The exposed surface of the MOF was sprayedwith the wetting solution, and the wet surface of the MOF was contactedwith the wet surface of the acrylic sheet. A heavy release liner wasapplied to the surface of the MOF to prevent damage to the MOF, and a3M™ PA-1 applicator (available from 3M Company, St. Paul, Minn.) wasused to squeegee the MOF down to the surface of the acrylic. Thisresulted in most of the wetting solution being expelled from between thetwo wetted surfaces. After this was done the second skin layer from theMOF was removed. Inspection of the applied MOF showed that the MOFsurface was much more irregular than the surface of the acrylic. Uponinspection again after 24 hours, the MOF surface was observed to becomparable in flatness to the acrylic sheet. This observed flatteningover time is consistent with residual wetting solution evaporating frombetween the two surfaces allowing the MOF to conform closely to thesurface of the acrylic. Even though the MOF conformed closely and stablyto the surface of the acrylic, it could be easily removed by peeling theMOF from the surface of the acrylic.

An imaging PBS was prepared by placing a small amount of Norland OpticalAdhesive 73 (available from Norland Products, Cranbury, N.J.) onto thesurface of the MOF. The hypotenuse of a 10 mm 45° BK7 polished glassprism was slowly placed into contact with the adhesive so that nobubbles were entrained in the adhesive. The amount of adhesive waschosen so that when the prism was placed on to the adhesive, there wassufficient adhesive to flow out to the edges of the prism, but not somuch adhesive to cause substantial overflow of the adhesive beyond theperimeter of the prism. The result was that the prism was substantiallyparallel to the surface of the MOF and separated by a layer of adhesiveof approximately uniform thickness.

A UV curing lamp was used to cure the adhesive layer through the prism.After curing, a section of the MOF that was larger than the prism andthat contained the prism was peeled away from the acrylic substrate.Removal was facilitated by bending the acrylic plate, thereby allowingthe rigid prism and MOF composite to separate more easily from theacrylic plate. Inspection of the prism/MOF composite showed that the MOFretained its flatness despite being removed from the acrylic plate.

The roughness parameters of the MOF were then measured as describedunder “Roughness Measurement Method” and are reported in the followingtable.

average stdev Ra (nm) 34 12 Rq (nm) 51 30 Rz (μm) 6.7 8.5A small amount of the Norland optical adhesive was applied to the MOFsurface on the prism/MOF composite. A second 10 mm 45° prism wasprocured and its hypotenuse placed in contact with the adhesive. Thesecond prism was aligned such that its principal and secondary axes weresubstantially parallel to those of the first prism, and the twohypotenuse surfaces were substantially coextensive. A UV curing lamp wasused to cure the adhesive layer so that the second 45° prism was bondedto the prism/MOF composite. The resulting configuration was a polarizingbeam splitter.

Example 2: PSA Method Using Heat and Pressure

An adhesive construction was formed by taking a sample of 3M™ OpticallyClear Adhesive 8141 (available from 3M Company, St. Paul, Minn.) andlaminating it to a reflective polarizing MOF using a roll laminationprocess. A piece of this adhesive construction was adhered to thehypotenuse of a glass prism similar to that used in Example 1. Theresulting MOF/prism composite was placed into an autoclave oven andprocessed at 60° C. and 550 kPa (80 psi) for two hours. The sample wasremoved and a small quantity of thermally curable optical epoxy wasapplied to the MOF surface of the MOF/prism composite. The prisms werealigned as in Example 1. The sample was then returned to the oven andagain processed at 60° C. and 550 kPa (80 psi), this time for 24 hours.The resulting configuration was a polarizing beam splitter.

Example 2A: Roughness Resulting from PSA Method Using Heat and Pressure

The roughness of MOF produced using the method of Example 2 wasdetermined as follows. A piece of MOF measuring 17 mm×17 mm waslaminated using a hand roller to a glass cube having a width of 17 mm.The glass cube had a flatness of about 0.25 lambda, where lambda equaled632.80 nm (a reference wavelength of light). The roll-laminated MOF wasannealed in an autoclave oven at 60° C. and 550 kPa (80 psi) for twohours. A Zygo Interferometer (available from Zygo Corporation,Middlefield Conn.) was used to measure the flatness of theroll-laminated MOF using light having a wavelength of lambda=632.80 nm.The Zygo Interferometer reported a peak to valley roughness, where atilt correction was used and no sphere correction was applied. The peakto valley roughness measured over the 17 mm×17 mm area was determined tobe 1.475 lambda or about 933 nm.

Example 3: PSA Method Using Vacuum

A piece of the adhesive construction of Example 2 was adhered to a glassprism in a manner similar to that in Example 2. The resulting prism/MOFcomposite was placed into a vacuum chamber equipped with a conventionalvacuum pump. The chamber was evacuated to around 71 cm (28 inches) ofHg, and of the sample held under vacuum for about 15 minutes.

The sample was removed from the vacuum chamber and the roughnessparameters of the MOF were measured as described under “RoughnessMeasurement Method.” and the measured values are reported in thefollowing table.

average stdev Ra (nm) 32 3 Rq (nm) 40 5 Rz (μm) 1.2 0.7

A second prism was attached to the prism/MOF composite using thetechnique and the UV optical adhesive of Example 1. The resultingconfiguration was a polarizing beam splitter.

Comparative Example C-1

A polarizing beam splitter configuration was created according to U.S.Pat. No. 7,234,816 (Bruzzone et al.). A piece of the adhesiveconstruction of Example 2 was adhered to a glass prism using a handroller thereby forming an MOF/prism composite.

The roughness parameters of the MOF were then measured as describedunder “Roughness Measurement Method” and are reported in the followingtable.

average stdev Ra (nm) 65 20 Rq (nm) 100 18 Rz (μm) 8.6 5.1

A second prism was attached to the prism/MOF composite using thetechnique and the UV optical adhesive of Example 1. The resultingconfiguration was a polarizing beam splitter.

Performance Assessment

The polarizing beam splitters of Example 1, 2, 3 and Comparative ExampleC-1 were assessed for their ability to reflect an image using aresolution test projector. A reference reflector consisting of one ofthe 45° prisms used in the other examples and operating as a totalinternal reflection (TIR) reflector was used to establish the bestpossible performance for the test projector.

A test target with 24× reduction was back illuminated with an arc lamplight source. Attached to the front surface of the test target was a 45°prism, identical to those used in earlier examples (and herein calledthe illumination prism). Light from the test target, travelinghorizontally from the source through the test target, entered one faceof the illumination prism, reflected off of the hypotenuse (via TIR) andexited the second face of the prism. The second face of the prism wasoriented such that the exiting light was directed vertically. Thevarious PBSs from the examples, as well as the reference prism wereplaced on top of the second face of the illumination prism. Thereflecting surface (MOF) in the PBSs as well as the hypotenuse from thereference prism were oriented such that the light reflecting from theMOF or the hypotenuse of the reference prism were directed forward andhorizontal. An F/2.4 projection lens obtained from a 3M™ SCP 712 digitalprojector (available from 3M Company, St. Paul, Minn.) was placed at theexit surface of the PBS or the reference prism and focused back onto thetest target, forming a kind of “periscope” layout.

This optical system was then used to assess the ability of eachdifferent PBS to resolve a test target while operating in a reflectionmode. In the system, an approximately 5 mm×5 mm portion of the testtarget was projected to about 150 cm (60 inches) diagonal. Within thisarea of the test target were multiple repeats of the resolution images.Five different identical repeats of the test target were assessed indifferent locations of the projected image: Top Left, Bottom Left,Center, Top Right and Bottom Right. Each test target was assessed todetermine the highest resolution that was clearly resolved. According tothe protocol, the maximum resolution was required to be resolved as wellas all resolutions below that level. There were instances wherelocalized distortions caused lower resolutions to not be resolved eventhough higher resolutions (in a slightly different location) wereresolved. The reason for this choice is that the full field and not justsmall areas must be resolved in order for the PBS to functioneffectively in a reflective mode.

Multiple samples of each Example were tested. Once the maximumresolutions were established for each location on each PBS, an averageand a standard deviation were computed for each type of prism (that is,for Examples 1-3, Comparative Example C-1 and the Reference prism.) An“Effective Resolution” was defined as the average minus two standarddeviations. This metric was determined from the data in “line pairs/mm”(lp/mm) and then expressed in terms of the size of the smallestresolvable pixel which was determined as ½ of the inverse of theEffective Resolution expressed in lp/mm. This definition accounts forthe fact that the resolution is only as good as the minimum resolutionacross the field. The Effective Resolution represents the maximumresolution that the particular PBS set can be expected to reliably(across 95% of the image) resolve.

Table 1 shows the results of the measurements of the different Exampleswithin this disclosure and Table 2 shows the resulting EffectiveResolution. As can be seen, the reference sample can resolve a 5 μmpixel. The PBS from Example 1 can also resolve a very nearly 5 μm pixel.Example 2 is able to resolve down to at least 12 μm and the PBS fromExample 3 can resolve down to 7 μm. All of these constructions should beadequate for at least some reflective imaging applications. On the otherhand, the PBS from Comparative Example C-1 is limited to resolvingaround 18 micron pixels, and would likely not be a robust choice for areflective imaging construction.

TABLE 1 Line Pairs/mm at Five Locations for Samples Top Bottom BottomTop Right Right Center Left Left Example Sample (lp/mm) (lp/mm) (lp/mm)(lp/mm) (lp/mm) Reference A 170.4 170.4 108.0 192.0 170.4 1 B 151.2170.4 120.0 151.2 120.0 1 C 151.2 151.2 108.0 120.0 151.2 1 D 151.2151.2 108.0 134.4 120.0 2 E 151.2 134.4 60.0 108.0 86.4 2 F 134.4 134.467.2 96.0 96.0 2 G 134.4 134.4 96.0 60.0 76.8 3 H 134.4 134.4 96.0 86.4120.0 3 I 134.4 151.2 108.0 96.0 96.0 C-1 J 151.2 134.4 48.0 60.0 76.8C-1 K 120.0 134.4 60.0 96.0 60.0 C-1 L 120.0 120.0 60.0 86.4 86.4 C-1 M134.4 120.0 60.0 60.0 86.4

TABLE 2 Effective Resolution of Exemplary Film Effective EffectiveAverage Std. Dev. Resolution Resolution Example (lp/mm) (lp/mm) (lp/mm)(μm) Reference 162.2 31.7 98.8 5.06 1 137.3 19.6 98.1 5.10 2 104.6 30.942.9 11.65 3 115.7 22.1 71.4 7.00 C-1 93.7 32.8 28.2 17.74The present invention should not be considered limited to the particularexamples and embodiments described above, as such embodiments aredescribed in detail to facilitate explanation of various aspects of theinvention. Rather the present invention should be understood to coverall aspects of the invention, including various modifications,equivalent processes, and alternative devices falling within the spiritand scope of the invention as defined by the appended claims.

The invention claimed is:
 1. A multilayer reflective polarizersubstantially transmitting a first polarization state and substantiallyreflecting an orthogonal second polarization state, the multilayerreflective polarizer capable of reflecting an imaged light toward aviewer or screen, the effective pixel resolution of the imaged lightbeing less than 12 microns after reflection from the reflectivepolarizer.
 2. The multilayer reflective polarizer of claim 1, whereinthe effective pixel resolution of the imaged light is less than 9microns after reflection from the multilayer reflective polarizer. 3.The multilayer reflective polarizer of claim 1, wherein the effectivepixel resolution of the imaged light is less than 6 microns afterreflection from the multilayer reflective polarizer.
 4. The multilayerreflective polarizer of claim 1 having a surface roughness Ra of lessthan 45 nm or a surface roughness Rq of less than 80 nm.
 5. Themultilayer reflective polarizer of claim 1 having a surface roughness Raof less than 40 nm or a surface roughness Rq of less than 70 nm.
 6. Themultilayer reflective polarizer of claim 1 having a surface roughness Raof less than 35 nm or a surface roughness Rq of less than 55 nm.
 7. Apolarizing beam splitter, comprising the multilayer reflective polarizerof claim 1 disposed between a first cover and a second cover.
 8. Thepolarizing beam splitter of claim 7, wherein the first and second covercomprise glass or plastic.
 9. The polarizing beam splitter of claim 7,wherein the first and second covers are attached to the multilayerreflective polarizer with a pressure sensitive adhesive, UV curedadhesive or an optical epoxy.